The present invention relates to immersion heaters or devices for heating liquid in a container. Such devices are used in industrial manufacturing processes, such as electroplating and the manufacture of semiconductors where it is necessary to maintain a bath of strongly acidic or caustic solution at constant elevated temperatures.
In certain process applications where it is required to have the liquid bath shallow relative to its periphery, it has been found desirable to have the immersion heater arranged in a generally flat planar array for location at the bottom of the liquid bath beneath the basket containing the articles to be immersed for providing rapid heating of the liquid and uniformity of temperature throughout the bath where extremely accurate bath temperature control is required. A known technique for constructing immersion heaters is that of utilizing a heater cable of coiled conductor suitably encased in a flexible plastic jacket or casing impervious to caustic or acidic baths, where the heater cable is wound about a suitable support in a desired configuration such as a spiral or serpentine array. Such a heater cable assembly is described in U.S. Pat. No. 4,158,764. Such known heater cables, although flexible, have been found incapable of being folded back or bent double about a relatively short radius compared to the outer diameter of the cable jacket, without destroying the coils of the heater element.
In providing the aforesaid type flat planar array heater cable assemblies having the greatest compactness, it has been found desired to find a technique for providing a continuous heater cable capable of being bent in a closely spaced serpentine arrangement requiring folding back or bending of the heater cable about a radius on the order of less than twice the outer diameter of the cable jacket. Such a closely spaced serpentine arrangement has heretofore required cutting the heater cable coil and splicing in by silver soldering circular elbows or corner fittings made of less resistive and more flexible material such as copper wire in order to accommodate such tight or short radius bends and thus have been costly and time consuming to manufacture.
In service applications employing planar array heater cable assemblies disposed with the plane of the array horizontal and parallel to the surface of the liquid, it has been found that a hazardous overheating occurs in the event the liquid level exposes the upper portions of the cable to air while power is connected to the heater. As the liquid level drops and exposes portions of the periphery of the cable along its length to air, local overheating of the cable jacket occurs with subsequent melting of the jacket and exposure of the liquid level during operation so as to expose portions of the heater cable to air.